This disclosure relates to a NOx adsorber catalyst for reduction of the amount undesirable emission components emitted in exhaust gases, an exhaust emission control system, and to methods of making thereof.
In order to meet exhaust gas emission standards, the exhaust emitted from internal combustion engines is treated prior to emission into the atmosphere. Typically, exhaust gases are routed through an exhaust emission control device disposed in fluid communication with the exhaust outlet system of the engine, where the gases are treated by reactions with a catalyst composition deposited on a porous support material. The exhaust gases generally contain undesirable emission components including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). As a means of simultaneously removing the objectionable CO, HC, and NOx components, various “three-way” catalyst compositions have been developed. When operating under lean-burn conditions (i.e., where the air-to-fuel ratio is adjusted to be somewhat greater than the stoichiometric ratio), however, typical three-way catalyst systems are relatively efficient in oxidizing unburned HC and CO, but can be inefficient in reducing NOx emission components.
To treat nitrogen oxides in the exhaust gases of engines operating under lean-burn conditions, NOx adsorbers can be added in exhaust lines along with three-way catalysts. A NOx adsorber typically contains a precious metal component that converts NOx species to nitrogen dioxide (NO2). Nitrogen dioxide in the presence of water (H2O) forms in situ nitrous acid (HNO3), a very strong acid. A conventional NOx adsorber formulation is an acid adsorber containing an alkaline oxide such as barium oxide (BaO). Barium oxide in the presence of water forms in situ barium hydroxide Ba(OH)2, a very strong base. The strong base adsorbs acids such as NOx and SOx sulfur oxide) that then reacts with hydrocarbons in the exhaust, especially double bonded species such as propylene, forming gaseous products (e.g., N2 (diatomic nitrogen or nitrogen gas), CO2 (carbon dioxide), H2O (water) and SO2 (sulfur dioxide)). A conventional NOx adsorber (e.g., one containing just an acid adsorber component), however, is usually sufficient only in an oxidizing atmosphere. In a reducing atmosphere (e.g., one containing hydrogen), ammonia is formed rather than the desired N2. The ammonia can react with water, forming ammonium hydroxide (NH4OH), a very strong base. Because of their low affinity for the acid adsorbing support, the ammonia and ammonium hydroxide can desorb from the acid adsorber before the conversion to N2 can be accomplished. Emission of ammonia and ammonium hydroxide to the atmosphere is not desirable.
There remains a need for fuel reformers and NOx adsorbers, particularly those that are effective in hydrogen containing atmospheres.